Diagnostic Circuit Check

The ECM provides the diagnostic logic to detect faults in the Diesel Electronic Control system that the ECM monitors or controls. The ECM, when it recognizes a fault, has the ability of turning the "SERVICE ENGINE SOON" light on and storing a code. If the condition corrects itself, the "SERVICE ENGINE SOON" light will be immediately turned off.

The ECM recognizes errors in engine speed, vacuum errors in the EGR vacuum loop, and electrical faults involving the 5-volt reference circuit. The ECM controls the following:

• Exhaust Gas Recirculation (EGR)
• Exhaust Pressure Regulation (EPR)
• Torque Converter Clutch (TCC)
• System Diagnosis
• Cold Advance & Glow Plugs

To allow proper engine control, the ECM monitors the following inputs:

• Engine RPM
• Manifold Absolute Pressure (MAP)
• Throttle Position Sensor (TPS)
• Vehicle Speed Sensor (VSS)
• Coolant Temperature Sensor (CTS)

All diagnosis should start with the "DIESEL DIAGNOSTIC CIRCUIT CHECK" chart. After any repair to the Diesel Electronic Control system, the diesel diagnostic circuit check procedure must be repeated.

1. This test checks for proper operation of "SERVICE ENGINE SOON" light. With ignition on and engine not running, the light should be on.
2. Grounding the diagnostic terminal will allow ECM to flash Code 12 and any stored codes. The light must go on and off for a proper code. If light goes from bright to dim, this is not considered a code.